The present invention relates generally to cellular communication systems, and, more particularly, to radio frequency (RF) transceivers in cellular communication systems.
A cellular communication system includes a mobile switching center (MSC), home location registers (HLRs), base station controllers (BSCs), base transceiver stations (BTSs), and user equipment (UE). A BTS facilitates communication between UEs and an MSC over a cellular network. The BTS includes an RF transceiver for transmitting and receiving RF signals to and from the UEs. Terms such as MSC, BSC, BTS, and UE are technology standard specific, and in this case are used in context of the Global System for Mobile communication (GSM) standard of wireless communication. For instance, the term BTS in the GSM standard corresponds to Node-B in the third generation (3G) and eNode-B in the fourth generation (4G) standards. The RF transceiver modulates a carrier wave by changing one or more characteristics of the carrier wave, viz. amplitude, frequency, and phase, based on an RF signal. The RF transceiver transmits a modulated carrier wave on a transmission medium by using an antenna.
FIG. 1 shows an RF transceiver system 100 that includes an RF transceiver 102 that is connected to an antenna 104 for transmitting the modulated carrier wave (hereinafter referred to as a “high-power RF signal”). The RF transceiver 102 includes a baseband processing unit 106, an RF integrated circuit (RFIC) 108, and a power amplifier (PA) 110. The baseband processing unit 106 includes a digital signal processor (DSP) 112, a system bus 114, a digital pre-distorter (DPD) 116, a direct memory access (DMA) system 118, a system memory 120, an event control module 122, and an antenna interface 124. The RFIC 108 includes an antenna interface 126, a data converter 128, and an RF mixer 130. The data converter 128 includes a digital-to-analog converter (DAC) 132 and an analog-to-digital converter (ADC) 134.
The DSP 112 performs logical and mathematical operations on digital data such as audio and video data and generates first baseband stream data. The DSP 112 is connected to the DPD 116, the DMA system 118, and the system memory 120 by way of the system bus 114. The DSP 112 is further connected to the event control module 122 and controls the event control module 122 by providing control trigger signals thereto. The event control module 122 generates trigger signals based on the control trigger signals to control the timing of events in the baseband processing unit 106 and the RFIC 108. The DPD 116 receives the first baseband stream data and generates pre-distorted first baseband stream data by multiplying the first baseband stream data with a set of coefficients from a look-up-table (LUT) stored therein. The set of coefficients are referred to as LUT coefficients. Further, the system memory 120 stores the pre-distorted first baseband stream data. The DMA system 118 and the DPD 116 are connected to the data converter 128 by way of the antenna interfaces 124 and 126. The antenna interface 124 receives the pre-distorted first baseband stream data from the DPD 116 and transfers the pre-distorted first baseband stream data to the antenna interface 126.
The DAC 132 receives the pre-distorted first baseband stream data from the antenna interface 126 and generates a first baseband signal. The RF mixer 130 that is connected to the DAC 132 receives the first baseband signal and generates a low-power first RF signal. The PA 110 that is connected to the RF mixer 130 receives the low-power first RF signal and generates a high-power first RF signal.
The RF mixer 130 receives the high-power first RF signal from the PA 110 and generates a second baseband signal. The ADC 134 receives the second baseband signal and generates second baseband stream data. The DMA system 118 receives the second baseband stream data by way of the antenna interfaces 126 and 124, and stores the second baseband stream data in the system memory 120. The DSP 112 compares the second baseband stream data with the pre-distorted first baseband stream data and selects alternate LUT coefficients from the LUT in the DPD 116. The DPD 116 multiplies the first baseband stream data with the alternate LUT coefficients to generate the pre-distorted first baseband stream data. The PA 110 receives the low-power first RF signal corresponding to the pre-distorted first baseband stream data and generates the high-power first RF signal at a higher power level.
It is desirable that the PA 110 achieves high efficiency and linearity. For instance, class A PAs are linear PAs but are expensive and thus unsuitable for cellular communication systems. Hence, inexpensive and non-linear PAs such as class AB, B, and C PAs are widely used. To maintain linearity of the PAs, digital pre-distortion technique is used. The DPD 116 performs a mathematical inversion of the high-power first RF signal of the PA 110. The DPD 116 is a non-linear module and the LUT coefficients have inverse characteristic of the high-power first RF signal. Thus, the pre-distorted first baseband stream data generated by the DPD 116 has an inverse characteristic of the high-power RF signal. When the non-linear PA 110 receives this pre-distorted first baseband stream data from the non-linear DPD 116, the PA 110 generates a linear high-power first RF signal. This process is referred to as digital pre-distortion.
For different RF signal conditions and cell radii of the BTS, it is essential to determine the ability of the DSP 112 to select alternate LUT coefficients for the corresponding RF signal conditions and the cell radii of the BTS. Thus, there is a need to calibrate the PA 110 for these RF signal conditions. The PA 110 is calibrated by adjusting the LUT coefficients of the DPD 116 during initialization of the RF transceiver 102 i.e., when the DSP 112 does not process actual digital data to be transmitted. The calibration of the PA 110 ensures that the PA 110 generates the high-power first RF signal at a desired power level when actual digital data is to be transmitted.
For calibrating the PA 110, the system memory 120 (or an external memory) stores reference baseband stream data. The DSP 112 shapes the reference baseband stream data to generate shaped reference baseband stream data by multiplying the reference baseband stream data with calibration patterns. The DSP 112 generates the calibration patterns that are mathematical polynomials having respective coefficients. The DPD 116 receives the shaped reference baseband stream data and generates a pre-distorted shaped reference baseband stream data. Finally, the PA 110 receives the pre-distorted shaped reference baseband stream data as a low-power reference RF signal and generates a high-power reference RF signal. Further, the DPD 116 repeats the process of digital pre-distortion and the DSP 112 adjusts the LUT coefficients of the DPD 116. Thus, at the end of the calibration process, the DSP 112 adjusts the LUT coefficients to ensure that the PA 110 generates a linear high-power RF signal.
A known technique in the art to calibrate the PA 110 loads the calibration patterns into the baseband processing unit 106 via external sources such as flash memory chips, local area network (LAN) and the like. As the calibration patterns are externally loaded into the baseband processing unit 106, this technique for calibration is not suitable in real-time applications. Further, as the DSP 112 accesses the system memory 120 multiple times to fetch the reference baseband stream data and the calibration patterns stored therein, data traffic on the system bus 114 increases. The increased data traffic results in higher power consumption of the RF transceiver 102.
An alternative solution is the use of the DSP 112 to generate the calibration patterns. However, the mathematical computation of generating the calibration patterns requires a high number of ‘million instructions per second’ (MIPS) (also referred to as “machine cycles”) of the DSP 112 that otherwise could be used for more important tasks such as digital data processing. Moreover, this solution also results in an increase in the data traffic on the system bus 114 and consequently an increase in the power consumption of the RF transceiver 102.
Therefore, it would be advantageous to have an RF transceiver that calibrates the PA in real-time with reduced number of MIPS of the DSP.